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The domination of Saturn’s low-latitude ionosphere by ring ‘rain’


Saturn’s ionosphere is produced when the otherwise neutral atmosphere is exposed to a flow of energetic charged particles or solar radiation1. At low latitudes the solar radiation should result in a weak planet-wide glow in the infrared, corresponding to the planet’s uniform illumination by the Sun2. The observed electron density of the low-latitude ionosphere, however, is lower and its temperature higher than predicted by models3,4,5. A planet-to-ring magnetic connection has been previously suggested, in which an influx of water from the rings could explain the lower-than-expected electron densities in Saturn’s atmosphere6,7,8. Here we report the detection of a pattern of features, extending across a broad latitude band from 25 to 60 degrees, that is superposed on the lower-latitude background glow, with peaks in emission that map along the planet’s magnetic field lines to gaps in Saturn’s rings. This pattern implies the transfer of charged species derived from water from the ring-plane to the ionosphere, an influx on a global scale, flooding between 30 to 43 per cent of the surface of Saturn’s upper atmosphere. This ring ‘rain’ is important in modulating ionospheric emissions and suppressing electron densities.

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Figure 1: The process of data acquisition.
Figure 2: Intensity of H3+ infrared emission as a function of position along Saturn’s noon meridian.
Figure 3: Comparison of H3+ intensity and the transparency of Saturn’s rings.

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The data presented here were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and NASA. The observations were made to support the Cassini auroral campaign. Ring profile data were provided by the Planetary Rings Node website18. Discussions within the international team led by T.S.S. on ‘Comparative Jovian Aeronomy’ have greatly benefited this work; this was hosted by the International Space Science Institute (ISSI). The UK Science and Technology Facilities Council (STFC) supported this work through the PhD Studentship of J.O’D. and grant support for T.S.S., H.M. and G.H.J.

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J.O’D. analysed and interpreted the data and wrote the paper. T.S.S., S.M. and K.H.B. proposed and designed the study and collected the data. H.M. greatly aided the analysis and interpretation of data. S.W.H.C. provided the magnetic-mapping model and magnetospheric information. G.H.J. provided ring-plane information. J.S.D.B. provided context from Cassini VIMS observations. All authors assisted in the interpretation of data and commented on the manuscript.

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Correspondence to J. O’Donoghue.

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The authors declare no competing financial interests.

Supplementary information

Supplementary information

This file contains Supplementary Text and Data 1-3 comprising: 1 Observational information, which elaborates on the details of the observations used to make the data; 2 Magnetic mapping Information, which discusses the details of the magnetic mapping used and also examines another leading magnetic field model as an alternative; and 3 Data, which shows the additional data plots. Supplementary Figures 1-9 are also included. (PDF 532 kb)

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O’Donoghue, J., Stallard, T., Melin, H. et al. The domination of Saturn’s low-latitude ionosphere by ring ‘rain’. Nature 496, 193–195 (2013).

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